Sorry, I forgot that keqtqiadv is not around at the moment, so he can´t answer questions.

From the FAQ thread:
How do Davis Cup and WTC work? When can you change nationality? How are captains chosen?

Nationality changes can only be done during the off-season (until the first tournament of the year is played). Captains are chosen based on the previous year's ranking (best ranked players from each country). Important: WTC/DC players aren't allowed to play challengers during these events.

So if you didn´t ask for a nationality change in the last off-season you can´t play for Poland